A piezoelectric energy harvesting device (hereafter, referred to as a ‘PEH’ device) generates the largest electric energy at the resonance frequency due to amplification of displacement occurring when the resonance frequency of the PEH device and the frequency of ambient vibrations meet each other.
The voltage generated from the PEH device is outputted in an AC (Alternating Current) type. A rectifier is used to convert the AC voltage into DC voltage. The rectifier is composed of four or two diodes, in a full-bridge type or a half-bridge type. A filtering capacitor reducing ripple of the DC voltage is connected to the rear end of the rectifier.
The output DC voltage of the filtering capacitor is used to drive an IC such as a memory, and MCU and RF transmitting/receiving devices. However, the electric energy obtained from small vibrations in an ambient environment is very small in magnitude, so the electric energy is not enough to drive an IC. Accordingly, methods of using stored power, if needed, by charging a storage device, such as a super capacitor, a secondary battery and a thin film battery, have been studied.
The electronic devices used at present in wireless sensor nodes more automatically operate by actively performing the functions or passively sensing or collecting information. For this configuration, a micro-energy supply device that can continuously supply power is required. Although batteries can perform this function, the amount of available energy decreases as time passes due to a limit in the storage capacity, and consequently, it becomes impossible to supply power to a wireless sensor node.